1. Field of the Invention
The present invention relates to a production method for a bushing used in an endless track mounted to vehicles.
2. Description of the Related Art
An endless track adapted to be mounted to vehicles includes, as shown in FIG. 1, a shoe 2, shoe connecting bolts 3, shoe nuts 4, rings 5 and 6, bushings 7, dust seals 8, and pins 9 as one structural unit thereof.
The bushing 7 used for an endless track is shown in FIG. 2 in an enlarged manner. For the endless track bushing, abrasion resistance is required at an inside surface 7a, an outside surface 7b and wall portions 7c adjacent the surfaces 7a and 7b, and strength and toughness are required at a core portion 7d of the wall to endure a load imposed on the bushing.
To satisfy those requirements, the following production methods of a endless track bushing have been proposed:
(a) A production method as proposed in Japanese Patent Publication SHO 52-3486, wherein case hardening steel (JIS (Japanese Industrial Standard): SCM415), which is a low carbon steel, is selected as the bushing material. The bushing material is carburized at portions near the surfaces thereof and is cooled in the furnace. Then, the bushing material is heated and quenched by oil, and then, the bushing material is tempered. The required hardness at the surfaces is obtained through the carburizing, and the required strength and toughness at the core portion are obtained through the quenching and tempering. This method will be called a first related art hereinafter. (b) A production method as proposed in Japanese Patent Application SHO 63-87338 proposed by the present applicant, (published as Japan 01-259,129) as shown in FIG. 3, wherein
a bushing material 10 of medium-carbon steel is carburized, and then the bushing material is cooled to an ambient temperature;
the bushing material is induction-heated beyond an outer carburized layer from an outside surface 10b thereof while the bushing material 10 is rotated about an axis 10a thereof whereby an outer effective hardened layer having a hardness greater than a specified effective hardness is formed;
the bushing material is induction-heated beyond an inner carburized layer from an inside surface 10c thereof while the bushing material 10 is rotated about the axis 10a and the outside surface is cooled by liquid whereby an inner effective hardened layer having a hardness greater than the specified effective hardness is formed and a tempered layer having a hardness less than the specified effective hardness is formed between the inner and outer effective hardened layer; and
the bushing is tempered at low temperatures. This method will be called a second related art hereinafter.
However, the first related art is relatively expensive as it takes a long time to carburize the bushing material because the case hardening steel includes is a low-carbon steel. The problem with the second related art is that it requires two steps in the induction heating because the bushing material is firstly induction-heated from the outside surface thereof and then from the inside surface thereof. Therefore, the hardening time is long.